Life, But Not As We Know It!

by Nicholas Mee on August 18, 2017

I remember many years ago watching an episode of Star Trek called The Devil in the Dark. The Starship Enterprise visits a mining community on the planet Janus VI where an unknown lifeform is playing havoc with the mining operations.

A silicon-based lifeform surrounded by its rock eggs, as envisaged in Star Trek.

The crew eventually track down the source of the disruption – a strange creature that looks like a rather angry pizza. It is actually a bizarre lifeform unlike anything encountered during their previous adventures. Spock performs a Vulcan mind-meld with the creature and is fascinated to discover that it is a sentient giant amoeboid being that burrows its way around the mountains living off solid rock. The reason for this unusual and rather crunchy diet is that, unlike the familiar carbon-based life on Earth, this is an organism whose fundamental chemistry is based on silicon.

Star Trek’s rock munching amoeba is far from the strangest lifeform dreamt up in science fiction. Here are a couple of my other favourite aliens.

The Black Cloud

The astrophysicist Fred Hoyle wrote a story in 1957 called The Black Cloud in which astronomers discover a vast gas cloud passing through the solar system. On further investigation this turns out to be a super-intelligent being that is wandering through the galaxy. When scientists work out how to communicate with the cloud, it expresses surprise that there are sentient beings inhabiting a solid planet.

Here Be Dragons

Robert L. Forward went even further in his remarkable novel Dragon’s Egg, describing the evolution of a civilization on a neutron star. There is no chemistry in the extreme environment of a neutron star, so these organisms are composed of hypothetical complex nuclear structures. This really is life in the ultra fast lane, as their biology revolves around nuclear physics instead of biochemistry. These fictional nanoscale creatures live their lives a million times faster than us.

Is There Life Out There?

Like the other macromolecules vital for life on Earth, DNA has a backbone of carbon atoms.

What do we know about the possible existence of life elsewhere? We have a very small sample with which to work, just one planet on which life is known.

Life on Earth is built around a biochemistry based on macromolecules that have a backbone formed of chains of carbon atoms. Our understanding of chemistry suggests that silicon is not a suitable building block for the complex chemistry required by life. Carbon seems to be uniquely suited to this role, so unfortunately silicon-based lifeforms will probably always remain fiction, for naturally occurring lifeforms anyway. It might be argued that artificial silicon chip based organisms will some day be created. Sentient clouds and nuclear organisms are also probably just entertaining fantasies, but who knows?

Living Is Easy

In all likelihood it is only possible for living organisms to evolve with a carbon-based biochemistry and a liquid water environment is essential. What we do know is that simple lifeforms arose early in the history of our planet. There is evidence for the existence of simple bacterial life as far back as 4 billion years ago, which is around 400 million years after the formation of the Earth, and perhaps as little as 100 million years after the formation of the oceans.

The interval between the arrival of conditions suitable for life and its appearance on Earth seems to have been a remarkably short geological time span. This suggests that life at the bacterial level arises easily and therefore should be common. If this is true, then we could expect to find life elsewhere in the solar system. There are a number of locations that might be good places to look.

Mars has been on the list for many years. In more recent times attention has focused on satellites of the gas giants, in particular Jupiter’s Europa and Saturn’s Enceladus.

The God of War

Illustration by Alvim Corrêa from a 1906 edition of The War of the Worlds by H.G. Wells.

The glaring red eye of Mars has long been associated with war, so it is no surprise that its supposed inhabitants should have been imagined as hostile towards us Earthlings, eager to get their tentacles around our necks.

The prospects of finding little green octopuses on Mars disappeared long ago, but there is still some hope of eventually finding microbial lifeforms. Mars is a cold and hostile world with little atmosphere today, but it may have been much more hospitable in the distant past. There is evidence that water once flowed on Mars, so it is conceivable that life emerged there. Bacteria survive in even the most challenging environments on Earth, so it is just possible that bacterial life has clung on for billions of years deep within the rock below the surface.

Europa

Europa is the fourth largest of the Galilean satellites of Jupiter, and is a little smaller than our Moon. It is thought to be a good candidate for extraterrestrial life as it appears to have a surface composed of ice, which seems to form a crust over a liquid water ocean. Europa is subjected to a continual flexing and straining by the gravitational pull of the giant planet Jupiter. This generates sufficient heat to maintain the liquid ocean. The ice sheets can be seen in the image below.

The surface of Europa is covered in ice-sheets. A liquid water ocean is believed to lie beneath the surface. Credit: NASA/JPL/University of Arizona.

NASA is currently planning the Europa Clipper Mission. We will learn much more about this icy moon and its possible habitability when it arrives at Europa in the mid 2020s.

Enceladus

An artist’s conception of the plumes of Enceladus. Credit: NASA/JPL-Caltech.

Enceladus is a moon of Saturn with a diameter of 500 kilometres. It is covered in fresh uncratered ice. In 2005 the Cassini probe discovered plumes of water venting from the south polar region of Enceladus, raising hopes that like Europa it may have a liquid water ocean beneath the surface.

Several other bodies in the solar system have been suggested as possible homes for living organisms. These include Jupiter’s giant moons Ganymede and Callisto, which are also believed to have subsurface oceans. Ceres, the largest of the asteroids. Neptune’s large moon Triton. Even distant Pluto. Perhaps the most intriguing possibility is Titan.

Titan

The Cassini mission to Saturn will soon come to an end after 13 years orbiting the gas giant. Cassini will take its final plunge into the planet next month on 15 September. The highlight of Cassini’s incredible mission came in 2005 when it released the Huygens probe to land on Titan, Saturn’s largest moon. Titan is slightly larger than the planet Mercury. It is one of the few bodies in the solar system where a probe has landed. This was feasible because unlike all the other moons in the solar system Titan has a thick atmosphere composed mainly of nitrogen with clouds of methane and ethane.

Artist’s conception of the Huygens probe parachuting through the atmosphere of Titan. Copyright: Emile-Raphael Franco.

The temperature on Titan is a rather chilly 180° Centigrade below zero. This is far too cold for liquid water to exist on the surface. Nonetheless, Titan has seas, lakes and rivers, but they are composed of methane and ethane, hydrocarbons that play a similar role on Titan to water on Earth. This has led to speculation that Titan might be home to some form of life. It is perhaps rather unlikely, but just about possible, that some sort of life could have evolved in this hydrocarbon rich environment. If so, it would have to be based on an unknown and exotic biochemistry. Last month, NASA scientists announced that a chemical known as acrylonitrile has been detected in the atmosphere of Titan. It has been suggested that acrylonitrile might be suitable for forming a novel kind of cell membrane.

Finding any sort of life elsewhere in the solar system would be sensational. It would suggest that Earth is not just an incredibly lucky one-off and that life is abundant throughout the universe.

 

Further Information

There is more here about NASA’s planned Europa Clipper Mission.

Click here for NASA’s announcement of acrylonitrile on Titan.

You can find updates on the final days of the Cassini mission here: The Grand Finale.

 

{ 1 comment… read it below or add one }

Chuck Ivie August 21, 2017 at 2:26 pm

Years ago I worked for CalTech as an astronomer at an observatory in California. We frequently held tours and one of the most commonly asked questions was “Is there life out there?’ So we talked about other planets and whether or not they could support life as we know it. One time a lady asked “What about life as we ‘don’t’ know it. Indeed, that is the question.

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